The amount and type of laughter between two people can potentially tell us much more than that they are sharing a joke. For example, friends laugh more than strangers, and shared laughter can be an indicator of sexual interest between a couple. But as onlookers, how well can we use the sound of laughter to make these kinds of inferences? A new study in PNAS is the first to investigate this and it turns out, regardless of our culture or where we live, we are pretty good at using laughter to identify the nature of other people’s relationships.

The researchers asked pairs of American, English-speaking undergraduates to come into the lab and talk about various topics, such as “bad roommate experiences”. Both individuals wore microphones, and their speech and laughter were recorded. Critically, some of the pairs of people were good friends, and some of them were strangers who had only met that day.

The researchers then took these audio recordings and extracted instances of “colaughter” between the pairs: that is, those times when both people had starting laughing within one second of each other, and there was no speech or other noise.

The researchers then asked participants recruited from all over the world to listen to these brief clips, and to make judgements about whether each pair of people were friends or strangers. There were 966 listeners from 24 countries across five continents, including India, Namibia, Peru and Slovakia.

The listeners were able to judge whether the laughter clips came from friends or strangers with a reasonable degree of accuracy – they got it right 61 per cent of the time, which statistically speaking is significantly better than if they’d simply been guessing. Most likely the listeners were tapping into the fact that the way we laugh with our friends sounds different from the way we laugh with strangers, including a shorter length of time for each burst of laughter, and more irregular pitch and volume. Amazingly, the listeners’ ability to judge which pairs were friends and which were strangers was very similar across cultures, including those with no familiarity with English. It doesn’t matter where you come from: it seems laughter is a language we all understand.

This skill has likely evolved because identifying others’ relationships from a distance was advantageous for our primate ancestors. For an outsider, it’s useful to recognise that two individuals are close to each other – it could signal that this is a united group worth joining, or if that’s not possible, that the pair represent a greater threat because they are closely allied.

While the origins of our sensitivity to laughter can be traced back millions of years, it’s a skill that’s still relevant to us today. Imagine starting a new job, and trying to work out the relationship between others in your office. Just as you will be attuned to other people’s body language and the content of their speech, you will likely deduce information from the way they laugh with each other. But remember, it works both ways. So next time you force a laugh with a colleague or acquaintance, don’t forget: for reasons stemming back to your primate ancestors, someone looking on might have you sussed out.

Post written by Dr Lucy Foulkes (@lfoulkesy) for the BPS Research Digest. Lucy is currently working as a postdoctoral research associate in Prof Sarah-Jayne Blakemore’s lab at the Institute of Cognitive Neuroscience on the MYRIAD project – a Wellcome Trust-funded project assessing the feasibility of teaching mindfulness in schools, and the ways in which mindfulness might promote mental health and resilience in adolescents.

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